Vertebrae interbody (spinal) fusion is a surgical procedure in which two or more vertebrae are joined or fused together. The objective of lumbar spine surgery, for example, is to stop the motion at a painful segment in the spine, thereby minimizing the pain and allowing the patient to increase their function. The theory is that if the joint does not move, the pain ceases. The fusion itself is achieved by creating a situation in which the body will try to repair itself by growing bone. As the bone grows between the two vertebral bodies, it fuses the painful joint together and eliminates the motion at that particular joint. In simplest terms a spinal fusion is a growing together of bone structures creating a solid bone bridge between vertebrae. Fusion surgeries typically require the use of bone graft to facilitate fusion. This involves taking small amounts of bone from the patient's bone (autograft), or from a donor (allograft), and then packing it between the vertebrae in order to “fuse” them together. This bone graft, along with a biomechanical spacer implant or interbody fusion device, will take the place of the intervertebral disc, which is partially or entirely removed in the process. Many state of the art cage technologies exist including those made of bone, titanium, polymer, and bioresorbable materials. Spinal fusion surgery is a common treatment for such spinal disorders as spondylolisthesis, scoliosis, severe disc degeneration, and spinal fractures, for example. Fusion surgery is generally considered only after extensive non-operative therapies have failed.
Three common fusion surgeries available include PLIF, ALIF and TLIF.
Posterior Lumbar Interbody Fusion (PLIF) concerns vertebrae access through an incision in the patient's back (posterior). The PLIF procedure generally involves MRI and CAT scans to determine what size interbody device (implant) the patient needs. Depending on the number of levels to be fused, a 3-6 inch incision is made in the patient's back and the spinal muscles are retracted (or separated) to allow access to the vertebral disc. The surgeon then removes the affected disc and surrounding tissue and prepares bone surfaces of adjacent vertebrae for fusion. Once the disc space is prepared, a bone graft or allograft, with an interbody device, is generally inserted into the disc space to promote fusion between the vertebrae. Additional instrumentation (such as rods or screws) may also be used at this time to further stabilize the spine.
Anterior Lumbar Interbody Fusion (ALIF) is similar to PLIF, however it is done from the front (anterior) of the body, usually through a 3-5 inch incision in the lower abdominal area or on the side. Once the incision is made and the vertebrae are accessed, and after the abdominal muscles and blood vessels have been retracted, the disc material is removed. The surgeon then inserts an interbody fusion device containing bone graft into the interbody space to stabilize the spine and facilitate fusion. Depending on the interbody device used and the condition being treated, additional instrumentation such as plates, rods and screws may also be used to further stabilize the spine.
Transforaminal Lumbar Interbody Fusion (TLIF) is a refinement of the PLIF procedure as a surgical treatment for conditions affecting the lumbar spine. The TLIF technique involves approaching the spine in a similar manner as the PLIF approach but generally more from the side of the spinal canal through an incision in the patient's back. This approach greatly reduces the amount of surgical muscle dissection and minimizes the nerve manipulation required to access the vertebrae, discs and nerves. The TLIF approach is useful for interbody fusion as it is generally less traumatic to the spine, is safer for the nerves, and allows for reduced access and endoscopic techniques to be used. As with PLIF and ALIF, disc material is removed from the spine and replaced with bone graft and an interbody fusion device inserted into the disc space. Instrumentation helps facilitate fusion while adding strength and stability to the spine.
A number of bone graft materials and interbody fusion devices may be used. The INFUSE® Bone Graft/LT-CAGE® Lumbar Tapered Fusion Device, for example, is a combination of an interbody device and an autograft replacement indicated for spinal fusion procedures in skeletally mature patients with degenerative disc disease (DDD) at one level from L4-S1, who may also have up to Grade I spondylolisthesis at the involved level. The INFUSE® Bone Graft/LT-CAGE® Lumbar Tapered Fusion Device is implanted via an anterior open or an anterior laparoscopic approach. INFUSE® Bone Graft, for example, represents a recombinant human bone morphogenetic protein-2 (rhBMP-2) formulation combined with a bovine-derived absorbable collagen sponge (ACS) carrier. To use INFUSE® Bone Graft, surgeons reconstitute the rhBMP-2 powder with sterile water and then apply it to collagen sponges. The sponges are inserted inside a pair of metallic cages, which are then implanted between the vertebrae. The thimble-like cages stabilize the spine and maintain the proper height between the vertebrae while fusion occurs.
The principal risk of these types of lower back surgery is that a solid fusion will not be obtained (nonunion) and further surgery to re-fuse the spine may be necessary.
Vertebroplasty is a minimally invasive, nonsurgical therapy used to strengthen a broken vertebra, for example, caused by a compression fracture. Vertebroplasty is generally accomplished by injecting polymethylmethacrylate (PMMA) cement into the vertebral body through a needle into the fractured bone. While PMMA has high mechanical strength, it cures fast and thus allows only a short handling time. Other potential problems of using PMMA injection include damage to surrounding tissues by a high polymerization temperature or by the unreacted toxic monomer, and the lack of long-term biocompatibility. Preissman U.S. Pat. No. 6,348,055, for example, reports that the use of syringes to deliver bone cement in vertebroplasty procedures. Preissman discloses using a screw-type high pressure injection device to provide an even injection pressure during delivery of the bone cement.
Calcium phosphate cements are conventionally prepared by mixing calcium phosphate powders of a special composition and a liquid, such as distilled water, for example, in a mortar to obtain kneaded cement which may then be filled into or applied to a defective portion of bone using a syringe or spatula, for example, and then allowed to cure. See, e.g., Mirtchi, et al., Calcium Phosphate Cements: Study of the beta-tricalcium Phosphate-Dicalcium Phosphate-Calcite Cements, Biomaterials, 11:83 (1990); F. C. M. Driessens, et al., Bioceramics, 10:279 (1997); K. S. TenHuisen, et al., Formation of Calcium-Deficient Hydroxyapatite from alpha-Ca3(PO4)2, Biomaterials, 19:2209 (1998).
The bone regenerative properties of calcium phosphate cements (CaPCs) is improved, for example, by the addition of growth factors, such as recombinant human Transforming growth factor-beta1 (rhTGF-beta1). Blom E J, et al., Transforming Growth Factor-Beta1 Incorporation in a Calcium Phosphate Bone Cement: Material Properties and Release Characteristics, J Biomed Mater Res., 59(2):265 (2002). See, e.g., Khan, S. N., et al., Use of Osteopromotive Growth Factors and Ceramics to Enhance Spinal Fusion, J Am Acad Orthop Surg., 13(2):129 (2005); Ruhe P Q, et al., Bone Inductive Properties of rhbmp-2 Loaded Porous Calcium Phosphate Cement, Biomaterials, 25(11):2123 (2004); Sandhu, et al., Spinal Fusion Using Bone Morphogenetic Proteins, Orthopedics. 2004; July; 27(7):717-8.
A need, however, continues to exist for minimal access methods to effect strong and stable interbody fusions without the employment of interbody devices.